1. Field of the Invention
The present invention relates to a clock regeneration circuit for use in a reception device of digital communication.
2. Description of the Related Art
In communication such as television and the like, a method is commonly employed in which image data and voice data are compressed using a coding system, such as one which complies to Moving Picture Expert Group (MPEG) standards or the like, prior to being transmitted.
In communication using such a coding system, the image data and the voice data of an image signal are separately coded and compressed in a transmission device to make data stream. Furthermore, the compressed and coded data streams are subjected to time division multiplexing to form packets, and it is the data packets which are then transmitted. At the other end, upon receiving the packets, a reception device extracts the data from each packet, and subjects the data to extension and decoding processing to obtain the original image and voice data.
At this point, during the decoding processing, the decoding must be performed based on and in conformity to a time reference at the time when the encoding processing was performed in the transmission device. Therefore, the reception device must synchronize a reference clock of the decoding processing section with that of the encoding processing section of the transmission device. For this synchronization processing, for example, in an MPEG2 system, a program clock reference (PCR) is included in each packet, as synchronizing information for calibrating a value of a system time clock (STC) as a time reference in the decoding processing section of the reception device in accordance with a time reference in the encoding processing section of the transmission device. This PCR is represented by a counted value of the reference clock of the transmission device.
The reception device includes a phase lock loop (PLL) circuit for generating the STC of a frequency in response to a control signal in order to calibrate the STC based on the PCR, and a counter which counts the STC generated by the PLL to obtain the counted value at a time when the PCR is received. A control section of the reception device obtains an error of the counted value of the STC with respect to the PCR, and controls a clock oscillation circuit so as to reduce error occurrences.
The reception device may be additionally provided with peripheral functions, such as that of transferring data to a memory buffer. The frequency of the reference clock of the control section (CPU) of the reception device is often increased to enable high speed control of such a peripheral function.
Commonly, the frequency of writing in a register which buffers a value of the PCR and a register which buffers a counted value of a STR do not agree with the reference clock of the control section (the CPU) of the reception device which reads the data from the registers. As a bit width of a bus of the control section of the reception device is usually 8 or 16 bits, when readout processing is performed based on the frequency of the reference clock of the control section of the reception device, a readout operation must be performed plural times (12 or 6 times) to read the value of the PCR and the counted value of the STC from the registers.
At this point, when the processing of the control section of the reception device is delayed for some reason while the value of the PCR and the counted value of the STC are being read, the next PCR is received and a problem results that the value buffered in the register is disadvantageously updated with a new value of the PCR and a new counted value of the STC, with the result that correct processing cannot be performed. In an attempt to address this problem, a method has been proposed in which a memory capacity of the register is increased to hold a plurality of values of PCRs and counted values of STCs so that the values are not updated during the processing. However, there is a problem that increasing the memory size of the register causes the overall chip size of the reception device to increase.